Current limiting fuse element

ABSTRACT

A current limiting fuse element characterized by a generally tubular electrically insulating casing having terminal means disposed adjacent to each of the opposite ends of the casing. A fusible element is disposed between and connected to said terminal means and is an elongated member having a plurality of spaced pairs of edge notches. The fusible element is tapered from the center towards each end.

United States Patent Cameron et al.

[451 Sept. 30, 1975 CURRENT LIMITING FUSE ELEMENT Inventors: Frank L. Cameron, North Huntingdon; John W. Staney, Penn,

both of Pa Assignee: Westinghouse Electric Corporation,

Pittsburgh. Pa.

Filed: May 10, 1974 Appl. No.: 468,779

U.S. Cl 337/159; 337/295 Int. Cl. l-IOlh 85/04 Field of Search 337/158, 159, 160, 161,

References Cited UNITED STATES PATENTS 907 5/1939 Lohausen 337/159 .577 l()/l953 Van Hoorn 337/295 X 3.569.889 3/l97l lwasaki et al. 337/159 FOREIGN PATENTS OR APPLICATIONS 623,292 4/l947 United Kingdom 337/295 Primary E.\'ami ner.l. D. Miller Assistant E.\'alizirier-Fred E. Bell Attorney, Agent, or Firm-L. P. Johns [5 7 ABSTRACT A current limiting fuse element characterized by a generally tubular electrically insulating casing having terminal means disposed adjacent to each of the opposite ends of the casing. A fusible element is disposed between and connected to said terminal means and is an elongated member having a plurality of spaced pairs of edge notches. The fusible element is tapered from the center towards each end.

5 Claims, 3 Drawing Figures US. Patent Sept. 30,1975

I m m F F 1: CURRENT LIMITING FUSE ELEMENT CROSS EFERENCES TOVRELATEDY APPLICATIONS This invention is related to those disclosed'in the application of Donald D. Blewitt et al, Ser. No. 407,335

filed Oct. 17, 1973, now U.S. Pat. No. 3,849,754-and the application'of Donald D. Blewitt, Ser. No. 407,336, filed Oct. 17, I973, now U.S.PatuNp. 3,868,619.

I BACKGROUND OF THE INVENTION 1. Field of the Invention: i

This invention relates to current limiting fuses and more particularly it pertains to the fusible elements.

2. Description of the Prior Art:

notch of each pair of notches laterally, opposite the other notch, the fusibleelement being tapered from the center toward each end, the apexes of the notches on one edge 'of the fusible element being aligned, the lines through the apexes on'opposite sides of the element being parallel, and the fusible elementbeingdisposed in a straight or helical path depending upon the voltage In the design of high voltage, current limiting fuses, the problems of large fault "current interruptions at higher .voltages can be solvedbya linear increase in length of the basic-fuse element. This is possible be cause the mechanics of fuse interruption of fault currents in the current-limiting range require that the fuse produce an arc voltage whichis approximately twice the peak of the system voltage. The arc .voltage which a fuse element will produce at high-currents is a function of the number of reduced cross sec,tions(notches or perforations) of the. element. This is in turn proportional to the length-of the fuse element. During high current fault interuptions, all reduced cross sections of the element melt in a nearly simultaneous fashion and prior to the melting of the unnotclied sections. The additive voltages of all of these arc-lengths results in desired overall effective arc-voltage. v

The interruption of low current faults, that is, faults which may take cycles or longer to melt-the fuse elements, is not as directly proportional to thelength of the fuse element. Fault clearing by the fuse at low currents is not accomplished by simultaneous, multisection melting of the element. Rather, for low.curre nt faults, the elements melt. at or near the center and burnback from that point until an.adequate length of element and sand has been fused. This fused silver-sand conglomerate, called a fulgurite, forms an insulator, the dielectric strength of which is a function of its length, diameter, and temperature.

The formation of an adequate, effective fulgurite for low current interruptions is closely tied in with the element rate of burn back. For higher voltage applications this rate of burn back often must be increased if the device is to clear. For example, whereas a notched silver strap of 0.250 X 0.005 inch will, in adequate length and immersed in sand, clear a 100 ampere fault at 5 KV, the same strap lengthened by a factor of three, will not clear 100 ampere at KV. The rate of burn back may be increased by uniformly decreasing the cross sectional area of the element, i.e. smaller elements.

While the use of smaller elements will facilitate the interruptions of higher voltages, such reduced element sizes have the adverse effect of a decreased full load current carrying ability. This means that higher voltage fuses must be made larger in size for a given current rating in order to accommodate the increased number of reduced size elements required.

SUMMARY OF THE INVENTION Generally, it has been found in accordance with this invention that an improved current limiting fuse structure comprises a generally tubular, electrically insulatof the current passing through the fuse.

The advantage of the fuse structure of this invention is that the fusible element is more effective on low cur.- rent clearing, whichdoesnotresult in a decrease in full load current carrying ability and which does not modify the high current fault clearing action of the fuse. The

full benefits of'a rapidly instituted arc voltage and attendant pronounced current limiting effect are thus preserved. 1

BRIEF DESCRIPTION v OF THE DRAWINGS bodyin g the invention.

1 DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 of the drawings, a current limiting fuse structure is generally indicated at 10 and is particularly adapted for lower voltages such as up to 2400 volts. The embodiment of the invention shown in FIG. 2 differs only in the configuration of the fusible element and is particularly adapted for high voltage applications, such as above 2400 volts. Inasmuchas the embodiments of both FIGS. 1 and 2 are substantially alike, only those parts which differ have different reference numbers. The fuse structure 10 includes a generally tubular casing or housing 12 which is formed from a suitable electrically insulating material that has sufficient structural strength to withstand the thermal conditions and internal pressures that may result during operation of the fuse structure, such as glass-reinforced epoxy or melamine resin.

Opposite ends of the casing 12 are closed off by similar terminal end caps or electrically conducting ferrules l4 and 16 which are secured in place by suitable means, such as the magnetic forming method which is described in detail in U.S. Pat. No. 3,333,336. Where desired, axially-extending electrically conducting studs 18 and 20 may be mounted on or integrally formed with the ferrules l4 and 16, respectively, to permit the mounting of the fuse structure in particular types of supporting structures. The fuse structure 10 also comprises annular terminal members 22 and 24 that are disposed between the opposite ends of thecasing l2 and the respective ferrules l4 and 16. Each of the terminal members 22 and 24 is formed from a suitable electrically conducting material, such as copper or a copper alloy, and includes a central opening with a tab portion or terminal 26 and 28 formed integrally at one side of the central opening which projects axially inwardly at one end of the associated casing.

In addition to the casing 12, the fuse structure comprises a fusible element 30 (FIG. 1) and 32 (HO. 2) as well as an electrically insulating support member or core 34 (FIG. 2). The fusible elements 30 and 32 extend between and are attached to oppositely disposed terminals 26 and 28.

As shown in FIG. 3 the fusible elements 30 and 32 may be formed from predetermined length of electrically conducting, fusible material, such as silver, of a flat, ribbon type and including a plurality of axially spaced points of reduced cross sectional area which may be formed by the V-notching the ribbon material from which the fusible element is formed on both sides at 36 and 38. Each fuse element 30 and 32 comprises first and second end portions 300 and 30b, 32a and 32b, which end portions are in electrical contact with their corresponding terminals 26 and 28 and extend between the corresponding terminal members 22, 24 and ferrules l4 and 16 in an electrically conducting manner.

in accordance with this invention as shown in FIG. 3, the fusible element 30, 32 is tapered from the center to the opposite end portions. More particularly, the opposite edges are tapered from the widest dimension at the center to narrow dimensions near opposite end portions. The several notches 36, 38 along opposite edges extend inwardly therefrom to aligned positions indicated by lines 40 and 42 which lines are parallel. In other words, the apexes of the notches 36 are aligned on the line 40 and the apexes of the notches 38 are aligned on the line 42. Each pair of notches 36, 38 are laterally disposed with respect to the longitudinal axis of the element 30, 32 and equally spaced longitudinally thereof from adjacent pairs of similar notches.

Although a fuse element 30, 32 was tested for an 8.7 KV fuse of 25 ampere rating, this invention is not limited to this rating only. The effectiveness of the low current clearing is achieved through the increase in rate of burn back due to the reduced cross sectional element at the tapered ends. The maintenance of a stipulated full-load current carrying ability is achieved by grading the temperature distribution along the element length. In a high voltage fuse, the elements 32 are relatively long, the heat sinks of the fuse ferrule or end overloads without sacrificing full loadability rating and pieces to be quite remote from the center of the fuse element. This results in the fuse element center being the hottest part of the fuse under load conditions, and a place where melting will initially occur under overload conditions. The fusible element of this invention retains a maximum element cross section at the fuse center and thereby does not decrease the minimum melting current associated with the element. The tapered sections, however, operate hotter than they would otherwise in a non-graded element. Thus, when the fuse does melt and begins to burn back from its center, the graded sections are consumed more rapidly both because of their reduced cross section and because their temperature is already higher and closer to that at which melting occurs.

Accordingly, the fusible element of this invention has an improved ability' to interrupt low current faults or without adverse effect upon current limiting interruption performance at high value of fault current.

What is claimed is:

l. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element disposed between and connected to said terminal means, the fusible element being an elongated member and having a plurality of spaced pairs of edge notches, one notch of each pair of notches laterally opposite the other notch, and the fusible element being tapered from the center toward-each end.

2. The fuse structure of claim 1 in which the apexes of the notches on one edge of fusible element are aligned.

3. The fuse structure of claim 1 in which lines through the apexes on opposite sides of the element are parallel.

4. The fuse structure of claim 1 in which the fusible element is disposed in a helical path.

5. The fuse structure of claim 4 in which an axiallyex-tending, electrically insulating support member is disposed in the casing, and the fusible element is mounted on the support member. 

1. A fuse structure comprising a generally tubular, electrically insulating casing, terminal means disposed adjacent to each of the opposite ends of said casing, a fusible element disposed between and connected to said terminal means, the fusible element being an elongated member and having a plurality of spaced pairs of edge notches, one notch of each pair of notches laterally opposite the other notch, and the fusible element being tapered from the center toward each end.
 2. The fuse structure of claim 1 in which the apexes of the notches on one edge of fusible element are aligned.
 3. The fuse structure of claim 1 in which lines through the apexes on opposite sides of the element are parallel.
 4. The fuse structure of claim 1 in which the fusible element is disposed in a helical path.
 5. The fuse structure of claim 4 in which an axially-extending, electrically insulating support member is disposed in the casing, and the fusible element is mounted on the support mEmber. 